Luminaires for providing general illumination to an area are well known and often used in outdoor lighting applications including roadway and sidewalk lighting, parking lot lighting, and residential area lighting. Luminaires having symmetric light distributions can comprise a light source disposed within an external optic, wherein the external optic is designed to provide the symmetric light distribution. In one architecture, for example, a luminaire can comprise a light source disposed within a bell-shaped external optic, wherein the bell-shaped external optic provides a symmetric distribution of light to an area.
In some applications, however, a higher degree of control over light distribution from a luminaire is desirable. In some roadway lighting applications, for example, it is desirable to use luminaires having asymmetrical light distributions operable to provide the roadway and shoulder areas with higher luminous intensity in comparison with non-roadway areas such as grassy medians. Moreover, in some residential outdoor area lighting applications, it is desirable to use luminaires having asymmetrical light distributions operable to mitigate or prevent light trespass.
Asymmetric light distributions from a luminaire can presently be created through several avenues. One avenue is to design an external optic operable to create an asymmetric light distribution when a light source is disposed within the external optic. Design of an external optic operable to provide an asymmetric light distribution, however, is often cost prohibitive due to time intensive design processes that can strain engineering resources. Moreover, the design of the external optic is usually restricted to addressing a particular lighting need thereby precluding use of the design in a variety of applications.
Another avenue for producing an asymmetric light distribution from a luminaire is to externally couple a secondary optic to a primary optic, wherein the secondary optic is responsible for creating the asymmetric light distribution. A reflective bell-shaped primary optic, for example, can have a refractive secondary optic coupled thereto, wherein the refractive secondary optic produces an asymmetric light distribution. Such an arrangement is illustrated in FIG. 1. As displayed in FIG. 1, a refractive secondary optic (102) is coupled to the bottom of a reflective primary optic (100). Creating an asymmetric light distribution with this architecture has significant disadvantages as the refractive secondary optic is likely to change the EPA wind loading of the luminaire while also increasing the weight of the luminaire. Furthermore, achieving designations such as IES Full-Cutoff becomes very difficult as the refractive secondary optic can cause uplight from the luminaire.
An additional avenue for producing an asymmetric light distribution from a luminaire is to block one or more portions of light from being transmitted by the luminaire. This avenue is disadvantageous since precluding portions of light from being transmitted by the luminaire reduces the luminous flux of the luminaire leading to inefficiencies and poor optical systems.
In addition to using asymmetric light distributions to focus the emitted light in desired directions and thus toward desired areas, it is also desirable that the illuminance of the light distribution from the luminaire (asymmetrical or otherwise) is uniform across the illuminated area. Traditionally, an external optic is designed such that the bottom section of a vertically oriented light source reflects at the highest desirable angle. A problem, however, arises with this construction when using high intensity discharge (HID) light sources. The luminance at the ends of an HID source are less than at other points along the source. As a result, the amount of high angle light provided by the luminaire is reduced, and light distribution from the luminaire is not as well-defined as desired. Ideally, the illuminance on the ground from a section of the external optic would be uniform across the illuminated area. A reduction in the amount of high angle light, nevertheless, diminishes illuminance as the edge of illuminated area is approached.